(732c) Cross-Linked Collidosomes Exhibit pH-Induced Morphogenesis

We present a biologically-inspired
strategy for developing functional materials with the capacity to morph.  We
utilized local mechanical cues to affect global shape changes in a network of
colloids called colloidosomes.  We used pH-responsive calcium alginate
particles (CAPs) that were ionically cross-linked with either 2% or 20% w/v
CaCl₂.  CAPs cross-linked with 2% w/v CaCl₂ exhibited approximately
twice the swelling of particles made with 20% CaCl₂.  Predictive
modeling showed that localization of responsive particles induced non-spherical
shape changes.  To verify the simulations, CAPs were dispersed into the aqueous
phase in a 1:1 ratio of 2% and 20% CaCl₂ while mineral oil was used in
the oil phase.  Colloidosomes were fabricated via a w/o emulsion stabilized by CAPs.
 Exposed carboxyl groups on the CAPs were cross-linked using a carbodiimide
coupling reaction.  We explored ethylene diamine (ED), butane-1,4-diamine (BD),
and hexane-1,6-diamine (HD) for achieving chemically cross-linked colloidosomes
with varying chain lengths between colloids.  Colloidosomes were then swollen
in phosphate buffer at pH 7.4 for 6 hr, and colloidosome morphology was
observed.  Uncross-linked colloidosomes did not increase in size or deviate
from a spherical shape after swelling.  Although cross-linked colloidosomes
with ED and BD resulted in increases in the colloidosome diameter after swelling,
the colloidosomes remained spherical.  Significant colloidosome shape changes
occurred when HD was used to tether CAPs together due to an increase in chain
length between cross-links.  Approximately 80% of the intact colloidosome
population cross-linked with HD exhibited large, non-spherical deformations.  The
formation of heart, flower, and dumbbell shapes were observed.  We quantified
the extent of global shape deformation by calculating the ratio of the surface
area after swelling to the initial surface area.  Colloidosomes cross-linked
with HD had a statistically significant change in area as compared to
colloidosomes cross-linked with ED.  Our findings suggest that local mechanical
stresses can generate new colloidosome isoforms, resembling morphogenesis in
biology.  We demonstrated that coordinated networks of heterogeneous subunits
may be used to design programmable materials.  This work may be useful in tissue
engineering, drug delivery and sensors where a nonlinear
response is desired.